Pharmaceutically active drugs can exist in different solid forms. For example, a drug may exist in different crystalline forms which have different physical and chemical properties.
Different physical properties can cause different crystalline forms of the same drug to have largely different processing and storage performance. Such physical properties include, for example, thermodynamic stability, crystal morphology [form, shape, structure, particle size, particle size distribution, degree of crystallinity, color], ripple behavior, flowability, density, bulk density, powder density, apparent density, vibrated density, depletability, emptyability, hardness, deformability, grindability, compressability, compactability, brittleness, elasticity, caloric properties [particularly melting point], solubility [particularly equilibrium solubility, pH dependence of solubility], dissolution [particularly dissolution rate, intrinsic dissolution rate], reconstitutability, hygroscopicity, tackiness, adhesiveness, tendency to electrostatic charging, and the like.
In addition, different chemical properties can cause different crystalline forms of the same drug to have largely different performance properties. For example, a crystalline form having a low hygroscopicity (relative to other crystalline forms) can have superior chemical stability and longer shelf-life stability (cf. R. Hilfiker, Polymorphism, 2006 Wiley VCH, pp 235-242).
One particular drug that is of great interest for use in treating cancer pain (and other acute, visceral, neuropathic and chronic pain pain disorders) is (1r,4r)-6′-fluoro-N,N-dimethyl-4-phenyl-4′,9′-dihydro-3′H-spiro[cyclohexane-1,1′-pyrano[3,4b]indol]-4-amine. This drug is depicted below as the compound of formula (I).
The solid forms of (1r,4r)-6′-fluoro-N,N-dimethyl-4-phenyl-4′,9′-dihydro-3′H-spiro[cyclohexane-1,1′-pyrano[3,4b]indol]-4-amine that are known so far are not satisfactory in every respect and there is a demand for advantageous solid forms.
In particular, there is a demand for solid forms of (1r,4r)-6′-fluoro-N,N-dimethyl-4-phenyl-4′,9′-dihydro-3′H-spiro[cyclohexane-1,1′-pyrano[3,4b]indol]-4-amine that have properties differing from corresponding solid forms or other solid forms of the diastereomer, i.e. of (1s,4s)-6′-fluoro-N,N-dimethyl-4-phenyl-4′,9′-dihydro-3′H-spiro[cyclohexane-1,1′-pyrano[3,4b]indol]-4-amine. Every property of one of the two diastereomers that differs from the corresponding property of the other of the two diastereomers may be useful for separating both diastereomers from one another. Isolation of pure solid forms of (1r,4r)-6′-fluoro-N,N-dimethyl-4-phenyl-4′,9′-dihydro-3′H-spiro[cyclohexane-1,1′-pyrano[3,4b]indol]-4-amine is very desirable from a pharmaceutical point of view.